The present invention relates to a CSMA wireless local area network having an antenna device and a terminal station.
A local area network (LAN) is developed in recent year. Because LAN is convenient even wired, and more if wireless, a high speed wireless LAN of transfer rate over 10 Mbps is considered for practical use.
One of access method in the high speed LAN is CSMA (carrier sense multiple access), used in the Ethernet. CSMA is a method in which each terminal station senses carrier to check the traffic on a channel for determining whether or not to send a frame. A terminal station that has frames to be sent on the network senses the carrier before transmission. If the carrier is not sensed at this time, the terminal station determines that other terminal stations are not sending frames and start transmission of its frames. If the carrier is sensed, the terminal station determines that another terminal station is sending frames and postpones its transmission.
In CSMA, the communication condition is that the carrier may be sensed between each of terminal stations. Therefore, as shown in FIG. 18, the communication area of a base station BS may be assumed a circle of radius R, the communication area that terminal stations T1 and T2 have to secure will be a circle of radius 2R.
However, the above formation area is under an ideal environment. In the practical environment, there may exist fluctuation on the transmission link, and obstacles that block radio wave such as walls, impeding the radio wave from reaching to the mated terminal station. The terminal station that the radio wave transmission signal may not be reached is referred to as a “hidden terminal station”. FIG. 19 shows an example of communication area that may have such a hidden terminal station. If there is a hidden terminal station, the carrier sense may not work effectively thereto, the frequency of collision of transmission frames may rise in the CSMA. Since the collision frames are rejected, the throughput characteristics that indicates the amount of transmitted information for a unit of time will be lessened.
In order to solve this problem, as disclosed in Japanese Unexamined Patent Publication No. H09-247187, a method has been devised in which a base station uses a sector type antenna to perform carrier sense for each sector and a signal selector switches sectors to alleviate the collision of frame packets. Although this method may avoid collision of packets, it may lessen the throughput of overall network because the communication area is still formed in a way that a hidden terminal station may exist.